Atomic oxygen (AO) plasmas used in microelectronic manufacture are generally described as glow discharge plasmas. Such low-pressure plasmas range from fractions of millitorr to about 10 torr and are weakly ionized with a typical electron density on the order of 10.sup.10 /cm.sup.3. Due to mobility differences, an electrically isolated surface exposed to the plasma develops a negative bias due to the greater mobility of electrons compared to positive ions. The potential difference between these surfaces and the bulk discharge is termed the plasma potential, typically 10 to 30 V. Such plasmas are initiated and sustained by electric fields which are produced by either direct current, (dc) or alternating current (ac) power supplies. Typically, ac frequencies of excitation are 100 kHz at the low end of the spectrum, 13.56 MHz on the radio frequency (rf) portion of the spectrum and 2.45 GHz in the microwave (mw) region. Plasmas may be interchangeably referred to as an electric discharge, gaseous discharge or glow discharge (the latter because they emit light). Plasma reactors available commercially, including flowing afterglow devices, plasma ashers, and the like, generally produce AO having an energy of less than about 0.05 eV.
The use of oxygen plasmas to strip photoresists is a well-known application of plasma etching in microelectronic device fabrication. This technology is an alternative to liquid etching, and is used to remove hydrocarbon resists after they have served their intended masking purpose. Liquid chemical stripping has several disadvantages including, for instance, the need to handle and dispose of large volumes of either hydrocarbon solvents or strongly oxidizing acids.
While resist stripping is a common use for oxygen plasmas, modified plasma discharges also have a role in multilayer lithography. Different plasma conditions, however, are required to etch anisotropically than those normally used in the isotropic resist stripping process: In stripping applications, the primary objective is the rapid removal of resist, without etching or damaging the underlying substrate. This is generally achieved at elevated pressures where the reactant supply is high and ion energy is low. Bilayer pattern transfer, however, requires selectivity between the silicon-containing and hydrocarbon-based materials, with etch anisotropy being the foremost requirement. This process is generally carried out in a reactive ion etching (RIE) mode, characterized by relatively low pressures and high ion bombardment energies. While ion bombardment is needed to increase the degree of etch anisotropy, it is also responsible for secondary damage to the underlying microcircuit.
As semiconductor devices have become increasingly integrated, the damage caused by fabrication processes presents an increasingly serious problem. The primary reason for damage formation is essentially the incidence of energetic particles, such as ions and UV photons, from the plasma to the substrate surface. These particles have energies larger than the damage-formation critical energy. Some ion-induced damage can be treated by removing damaged layers, including techniques such as wet chemical etching, low voltage reactive ion etching or thermal annealing in the presence of reactive gases. However, each additional step decreases the anisotropy of the total process since these removal steps are inherently isotropic in character. In addition, a thermal annealing step, which necessarily takes place after the plasma etching in typical production processes, is not desirable since it changes the boundary shape of doped areas due to thermal diffusion of doping elements. Some effort has been directed to reducing damage caused by energetic ions by decreasing the ion penetration depth by using lower ion energy or heavier ion species. Alternatively, protective coatings on the substrate are used which absorb energetic particles.
An additional serious problem resulting from use of ion enhanced thermal AO processes is that of by-product impurity contamination. Relatively lower volatility reaction products can remain on the substrate as impurities or, plasma related impurities can penetrate the substrate during RIE etching either as a result of direct implantation or diffusion.
Accordingly, it is desirable that a method be developed for anisotropic etching in semiconductor manufacture utilizing oxygen plasmas essentially free of energetic ions and photon particles. Such particles are ordinarily required to achieve an anisotropic etch with atomic oxygen plasmas in the prior art, but cause secondary damage to the underlying substrate. Furthermore, the reaction products should be highly volatile and easily removed from the etched substrate.
Enhanced reactivity of the chemical component at a constant ion energy leads to a higher sputter yield and may reduce material damage. J. A. Skidmore et al. Journal of Vacuum Science Technology, pp. 1885-1888, (November/December, 1988) reports that if the chemical etching component is highly reactive and separately controlled from the physical component (ion beams), then very high reactive sputter yields can be obtained for etching GaAs with a degree of anisotropy that can be varied over a wide range by varying the microwave power level and the ion beam current density. This etching technique should be capable of producing smooth, low-damage structures for applications in optoelectronics.
H. Yamadan, et al. Journal of Vacuum Science Technology, pp 175-180, (March/April, 1989) discloses the oxygen plasma etching resistance of a plasma polymerized organometallic film deposited on a substrate surface by downstream plasma deposition using gaseous mixtures of an organic monomer (C.sub.3 H.sub.6) and a tetramethyltin or tetramethylsilane monomer. High oxygen plasma etching resistance was obtained on films containing a few percent metal. It was observed that the metal-oxidized layer was formed on the surface during plasma etching.
M. W. Geis et al., Journal of Vacuum Science Technology, pp 1928-1929, (July/August, 1987) and pp 363-365, (January/February, 1987) discloses two dry etching techniques. A first technique utilizes a combined Ar.sup.+ beam from an ion source and a directed flux of chemical reactive species from an effusive source and has high aspect ratios for GaAs and other materials. A second technique does not require an ion beam but only uses a directed flux of chemically reactive species that consist primarily of radicals.
Suzuki et al., Journal of Applied Physics. pp 3697-3705, (Oct. 1, 1988) discloses anisotropic etching of polycrystalline silicon with a hot Cl.sub.2 molecular beam. The etching was said to be almost damage free.
Suzuki et al., Journal of Vacuum Science Technology, pp 1605-1606 (July/August, 1987) discloses silicon etching with a hot SF.sub.6 beam. The advantages of using a hot molecular beam are said to reside in molecules having stored vibrational energy which is generally much lower than the displacement energy of a silicon crystal. The vibrational energy is lost when a hot molecule reflects off a cold surface. Therefore, an anisotropic etching is expected, and the molecule can supply both reactive material and energy to a surface.
U.S. Pat. No. 4,838,817 to Outlaw discloses a method for producing an atomic oxygen beam where an essentially pure beam of atomic oxygen is produced in a terrestrial laboratory at sufficient flux and energy levels to simulate conditions of low Earth orbit. The method comprises a material which dissociates molecular oxygen and produces atomic oxygen. Heating and excitation releases atomic oxygen into a beam.
U.S. Pat. No. 4,780,608 to Cross et al. discloses a laser sustained discharge apparatus for the production of intense beams of high kinetic energy atomic species. Oxygen atoms having velocities in excess of 3.5 Km/s can be generated for the purpose of studying interaction with materials suitable for spacecraft protection.